Batteries are unable to continuously supply a stable voltage, so systems using batteries as power source, e.g. portable electronic devices, need to be equipped with a buck boost voltage regulator that is capable of switching between a buck mode and a boost mode. FIG. 1 is a circuit diagram of a buck boost voltage regulator, in which a power stage 10 has a power switch SW1 connected between the power input terminal Vin and a node N1, a power switch SW2 connected between the node N1 and ground GND, an inductor L connected between the node N1 and a node N2, a power switch SW3 connected between the node N2 and ground GND, a power switch SW4 connected between the node N2 and the voltage output terminal Vout, and a capacitor CL connected between the voltage output terminal Vout and ground GND, voltage divider resistors R1 and R2 divide the output voltage Vout to generate a feedback signal VFB, and a control circuit 12 has an error amplifier 14 to amplify the difference between the feedback signal VFB and a reference voltage Vref to generate an error signal VEA, a pulse width modulation generator 16 to generate a pulse width modulation signal PWM according to the error signal VEA, and a control logic circuit 18 to generate control signals VA, VB, VC and VD according to the pulse width modulation signal PWM to switch the power switches SW1, SW2, SW3 and SW4 respectively, to convert the input voltage Vin to the output voltage Vout. U.S. Pat. No. 7,265,524 issued to Jorden et al. proposed a control method for a buck boost voltage regulator, which monitors the output voltage Vout of the buck boost voltage regulator when the buck boost voltage regulator operates in a buck-boost mode, and when the output voltage Vout goes above a high voltage threshold, switches the buck boost voltage regulator to a sleep mode. In this sleep mode, the power switches SW1, SW2, SW3 and SW4 are all open to stop charging the inductor L of the buck boost voltage regulator and thus the output voltage Vout decreases slowly. Till the output voltage Vout goes below a low voltage threshold, the buck boost voltage regulator is switched back to the buck-boost ode in which the inductor L is continuously charged/discharged to regulate the output voltage Vout.